The present invention, which is an expansion on inventions described in a previously filed application, entitled Uniform Illumination System filed on Dec. 14, 2001, Ser. No. 10/319,800, and which is incorporated by reference herein, is concerned generally with a thin and compact multi-layered optical system and method for generating well-organized output illumination from a one or two-dimensional array of discrete light emitting diodes (LEDs), the output light spread uniformly over the system's aperture while emanating from a uniquely multi-layered system comprised of reflecting bins and elevated light directing films. The present invention focuses centrally on the beneficial interactions between the geometric parameters of a thin array of metallically-reflecting bins, each having four tapered sidewalls meeting at an input aperture containing an LED, and the geometric parameters of orthogonally oriented prism sheets (and/or polarization converting sheets) placed above them. The previous invention described the basic geometric configurations of such multi-layers, while the present invention explores their performance differences, and in doing so, sets forth two specific embodiments related to directed LED lighting and illumination, as well as adding means for additional efficiency gains by the external recycling of otherwise wasted light. The first LED light source array embodiment trades optical efficiency to achieve output beams having the highest practical density of lumens, making very high-power illumination applications such as occur in video projectors practical at the soonest opportunity. In this non-etendue-preserving embodiment, interactions between reflecting bins and elevated prism sheets, polarization-converting films and/or micro-lens arrays cause beneficial spatial overlap of bin outputs that increase the array's effective lumen density. The second LED light source array embodiment achieves highest possible optical efficiency, allowing high-brightness illumination applications using the fewest possible LEDs and/or the lowest amounts of electrical power. In this etendue-preserving embodiment, shaped reflecting bins are combined with elevated micro-lenses and polarization converting films to manipulate the illumination pattern especially for square or rectangular illumination targets. Accordingly, the field of illumination produced by the particular optical systems containing these multi-layered emitting arrays provide a suitable illuminating beam for projecting an electronic image (as from an LCD or DMD) onto a screen, or the illumination itself composed of separately-controlled image pixels, the sum of which at any instant forming a spatially modulated image to be viewed directly, as in LED image displays for signage and video. The field of directed illumination may also be used as a means of general illumination, as in lighting fixtures and luminaries. More particularly, the multi-layer optical system that achieves this favorable performance consists of a heat extraction layer, an electronic back plane containing a regular one or two-dimensional array of electronically interconnected LEDs (preferably flip-chip style), an micro-fabricated array of contiguous (or nearly contiguous) reflecting bins with shaped or plane tapered sidewalls, one bin surrounding each LED (or group of LEDs), and a sequence of at least one additional optical light directing layer positioned above or at a preferred spacing from the reflecting bin apertures, the layer construction designed in conjunction with the geometry of the underlying reflecting bins, so as to maximize the light source array's output power and field coverage within a particular angular range, or within a particular angular range and polarization state. An additional layer or layers, in configurations that needing some additional diffusive mixing, can be conventional light spreading materials such as holographic diffusers, lenticular diffusers, lens arrays, bulk or surface scattering diffusers, opal glass, or ground glass, added to improve spatial uniformity.
Currently available illumination systems capable of achieving equivalent brightness uniformity (and lumen density) using only conventional optical elements, do so with at least 2 times fewer lumens per square millimeter, less efficiently (in terms of brightness), and in considerably thicker and less well-integrated packaging structures. Currently available LED illumination systems use arrays of discretely packaged LED devices, or LED chips on interconnection planes disposed below conventional refractive optical elements (whose effective optical collection range is limited). By comparison, the uniqueness of the present invention relates to the fact that its compartmentalized packaging layer and its cooperatively designed optical over-layers are both made to be continuous elements for the entire array—and whose choice of materials and their geometry achieves significantly enhanced performance. Designing the reflecting bins and the optical layers above them interactively, and by means of a realistic and experimentally validated computer model, is found to maximize optical output compared with more conventional designs. The increase in the performance of such LED light source arrays is not an obvious step despite previous use of LEDs in arrays, in reflective packages, and in conjunction with many types of conventional secondary optical elements.
Such compact LED illumination systems are of primary interest for the projection of images onto screens from such spatial light modulators as reflective and transmissive LCDs and DMDs. LED illumination is considered superior to the commonly used discharge lamps with regard to operating lifetime, which increases nearly 100-fold, and also because the conductive heat generated in the LEDs is easier to extract than the radiative heat given off by a gas discharge. Using LEDs in place of short-arc discharge lamps, however, is not straightforward for several reasons. Discharge lamps generate 60 (white) lumens per watt at 130-150 watts, and today's projection systems have rather low end-to-end optical efficiencies in the range of 15% and less. Imagining the use of today's best high-power LEDs at light levels of 7000 to 9000 lumens seems quite difficult, given that best emission efficacies are only in the range of only 15-25 lumens per watt. What's more, manufacturing economies keep typical LCD and DMD image apertures less than 1.2″ on the diagonal, and such devices cannot make effective use of light at angles above +/−12 degrees. This means that the total effective illumination area for the +/−90 degree emitting LEDs has to be less than 19.28 mm2, or for the standard image 4:3 aspect ratio, less than a rectangular area 5.07 mm by 3.80 mm. While such jumbo chips might become available in the distant future, the largest chips known today are square and not yet larger than 1 mm or 2 mm on an edge (as manufactured by LumiLeds, San Jose, Calif.). Even were such jumbo chips available, the challenge would still be to convert all its generated lumens to the +/−12-degrees needed in practical image projectors with high enough efficiency and spatial illumination uniformity. At today's best LED lumen density of 50 lumens/mm2, the total lumen yield from such a small illumination aperture would not be nearly enough after projection system transmission losses to reach competitive projection screen powers, which must be at least 1000 white-field lumens for many product applications of commercial interest.
The basic approach for overcoming this limitation has been described previously and involves using spatially separated high lumen density multi-layered arrays of separated red, green and blue LEDs, these arrays arranged and designed to concentrate their output emissions to a particular range of narrowed output angles (and polarization states) that can be handled efficiently by the conventional optics of a modern image projection system. Once so-created and integrated with the respective reflective or transmissive LCDs (or reflective digital micro-mirror devices, DMDs or DLPs as trade marked by Texas Instruments), the LED array output beams are mixed using the standard dichroic mixing cubes that allow the single-colored beam apertures to be superimposed on each other.
The present invention extends the basic approach to specific very high lumen density illuminator embodiments that enable with the best of the forthcoming high-power flip-chip LEDs, a wide range of compact and practical image projectors.
The present invention also extends to very low power, potentially hand held image projectors suitable for battery operation.
Such compact high lumen density LED illumination systems are also of interest for certain traffic signals and alerts, interior lighting, street lighting, stage and theatrical lighting, automotive head and tail lighting, safety warning lights, the backlighting of LCD screens and certain fiber optic medical illuminators.
These same compact high lumen density multi-layered illumination systems may be adapted for their intrinsic ability to display pixelized images directly, where in each reflecting bin within the light source array involved contains one each of a red, green and blue LED, and wherein every LED in the array is individually-addressed.